Lindane (gamma-hexachlorocyclohexane), a widely used insecticide, may be found at low concentrations in the human diet. Our previous studies have shown that 3-day lindane exposure at doses of 20 and 40 mg/kg/day can induce a residual progenitor cell damage which was examined one week following the second and fourth biweekly whole body irradiation (WBI 200 rads). The present studies were designed to determine whether longer lindane exposure at lower doses would also cause residual marrow effect in mice. Seven-week old male B6C3Fl mice were given lindane daily by gavage in corn oil for 10 consecutive days at doses of 0, 10, or 20 mg/kg body weight. This exposure level did not cause clinical signs nor changes in body weight, but caused a dose-dependent decrease in marrow cellularity, committed granulocyte-macrophage progenitors, and more pluripotent stem cells which returned to control values by four weeks. Then mice were subjected to two WBI (100 rads) at four and nine weeks following cessation of lindane treatment. This level of irradiation has indicated to cause a transient drip in marrow progenitor numbers. Controlled and exposed mice were examined at one and six weeks following the last irradiation which was 10 and 15 weeks following the final lindane exposure. The previously lindane exposed mice had lower progenitor cell numbers and slower recovery from the irradiation. These results demonstrate that lindane exposure in mice at levels that do not cause clinical toxicity can induce residual bone marrow toxicity. Furthermore, the stem cells are studied by assessing the ability of normal marrow cells transplanted to restore marrow injury in mice pretreated with lindane plus irradiation. (These results were published in Fed. Proc. 1991.)